Steam – How to Limit Download Bandwidth?

In your library list, right click on the game you wish to cancel the download for and click 'Delete Local Content'. This will remove the data that has already been downloaded and remove the game from the download list.

If there are any special network configurations you have to set on your computer, you should ask your internet service provider.

Otherwise, in all likelihood, you will just have to pay more for a faster internet connection (assuming you have a choice and your local infrastructure supports it).